JPS58835B2 - induction heating cooker - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an induction heating cooker, and particularly to its no-load detection means or small article load detection means.

Conventionally, in induction heating cookers, small metal objects such as spoons and forks are not heated because the flame cannot be seen and there is a risk of burns if a person touches small metal objects such as spoons and forks. A means of preventing heating was required.

Conventional small load detection means for detecting small metal objects include detecting the input current of an inverter device that generates a high-frequency magnetic field using high-frequency current, or detecting the regenerative diode current or conduction period of the regenerative diode in the inverter circuit to determine the load status. There are means to detect.

However, it has the disadvantage that the circuit is complicated, has a large number of parts, and is expensive.

The present invention provides a small object load detection means with a simple circuit configuration and low cost.

A detailed explanation will be given below according to the drawings.

FIG. 1 shows an embodiment of an induction heating cooker according to the present invention, in which AC voltage is applied to a rectifier circuit 2 from a low frequency AC power supply 1,
Output DC terminal of rectifier circuit 2 that converts to DC voltage■1. ■
An input capacitor 3 is connected between the two.

Also, the output current terminal ■1. (2) is connected to the input terminal of the inverter circuit 4, and the heating coil 5 is connected to the output terminal of the inverter circuit 4.

Normally, the heating coil 5 also serves as a resonant inductor for the inverter circuit 4.

The inverter circuit 4 is controlled by a control circuit 6 and constitutes an inverter device.

In order to detect the terminal voltage Vc of the input capacitor 3, the control circuit 6 connects the DC terminals ■1. It has input voltage detection terminals 600a and 600b connected to V2, and has a control output terminal 601 that controls conduction of a power semiconductor switching element included in the inverter circuit 4.

Figure 2A shows the terminal voltage Vc of the input capacitor 3 when using an ordinary iron making pot with a large induction heating output, and Bζ'! /J\This is Vc in case of material load.

The impedance after the inverter circuit 4 can be considered as an equivalent resistance load, and when the induction heating output is large, the equivalent resistance is small, and when the load is small, it is large.

Therefore, the input capacitor 3 is normally selected to have a value of 10 to 30 μF in order to improve the power factor, so that the terminal voltage Vc usually has a full-wave rectified waveform under normal pot load.

However, in the case of a small load, the power factor becomes poor and the discharge voltage of the input capacitor 3 remains even near AC zero voltage, so that a voltage Vcmin as shown in FIG. 2B is generated.

The present invention provides a small load detection circuit that determines that the load is a small load when the voltage near AC zero voltage exceeds a certain level.

FIG. 3 shows a block diagram of a control circuit according to the present invention, FIG. 4 shows a specific example of the circuit, and FIG. 5 shows waveforms of various parts thereof.

In Figure 3, DC terminal ■1. (2) Input voltage detection terminal 600a of the inverter device connected to 2.

600b as an input, its output signal v8 is applied to a comparator circuit 62, and its output signal V is applied to an oscillation start/stop control circuit 63 that controls oscillation start/stop of the inverter circuit 4.

Voltage detection circuit 61 detects the terminal voltage level of input capacitor 3.

A resistor 610, a Zener diode 611, and the base of a transistor 612 are connected in series from a DC (G) terminal 1, and the emitter of the transistor 612 is connected to ground.

The DC (to) terminal ■2 is commonly connected to the ground of the control circuit 6.

An anti-parallel diode 613 is connected between the base and emitter of the transistor 612, and a collector resistor 6 is connected to the collector.
Connect 14.

The cathode terminal of the Zener diode 611 is a direct current (T)
The transistor 612 is connected to the terminal (1) side, and when the Zener voltage is higher than Vz, the transistor 612 is turned on.

A transistor 615 is connected to the collector of the transistor 612.
The resistors 617 and 6 are connected by a diode 616 connected in series with the collector of the transistor 615.
18 and a timer circuit of capacitor 619.

Namely. As shown in FIG.
As a result, the charge in the capacitor 619 is discharged, and the voltage of the capacitor 619 becomes almost zero.

When Vc is higher than the Zener voltage Vz, a capacitor 619 is charged via a high resistance 617 and a low resistance 618.

Since the charging time constant is longer than the period of a half wave of a low frequency AC radio wave, the capacitor 619 voltage is typically at a low level for iron pots.

In other words, normally when using an iron pot, the input voltage VC is lower than the Zener voltage every cycle, and the transistor 61
5 to discharge the capacitor 619 every cycle.

However, with a small load, Vc does not become lower than the Zener voltage Vz, the transistor 615 is turned off, and the capacitor 619 continues to be charged by the high resistance 617, becoming higher than the threshold value vth of the inverter 620, and the output of the inverter 620 V becomes L level.

(2) becomes L level, the oscillation start/stop control circuit 63 puts the inverter circuit 4 into the oscillation stop state.

When starting oscillation, the input voltage Vc is always higher than the Zener voltage Vz, and the voltage of the capacitor 619 is the control circuit voltage Vcc.
, and the output signal Vp of the comparator circuit 62 is at L level.

Therefore, when oscillation is started, the oscillation start and stop circuit 63
It is necessary to lower the capacitor 619 voltage, turning on the transistor 631 through the base resistor 630 and rapidly discharging it through the diode 632 and low resistance 618.

At the start of oscillation, a narrow pulse signal is added to the input signal (1) to the base resistor 630.

FIG. 6 shows another embodiment of the present invention, and FIG. 7 shows waveforms of various parts thereof.

This embodiment is provided with a zero voltage switching circuit 64 and a zero point voltage detection circuit 65 to determine whether the input capacitor 3 terminal voltage Vc at AC zero voltage is at a set level.

That is, from the AC input terminal, a full-wave rectified waveform as shown in FIG. .

The emitter of the transistor 642 is connected to the DC (to) terminal ■2.

A diode 643 is connected antiparallel to the base and emitter of the transistor 642.

Divided resistors 650a and 650b are installed between DC terminals v1゜■2.
are connected in series to extract a voltage Vd proportional to the input capacitor 3-terminal voltage Vc.

The collector of the transistor 642 and the dividing resistor 650a, 6
By connecting the connection points 50b, a voltage vdp proportional to the terminal voltage Vc is generated at zero AC voltage as shown in FIG. 7 Vd.

The above is the operation of the zero voltage switching circuit 64.

Comparison circuit 651 includes a latch circuit therein, and when signal Vd becomes higher than threshold value vth, output signal Vp becomes H level.

As described above, according to the present invention, the load state of the induction cooking device is detected by detecting the terminal voltage near zero voltage of the input capacitor of the inverter circuit, in other words, the level of the DC voltage of the rectifier circuit. Therefore, it is possible to construct an inexpensive small object detection means with a simple circuit configuration.

Further, according to the present invention, when the low frequency AC voltage increases,
Since the input capacitor terminal voltage increases and the power consumption of the inverter circuit also increases, a heating operation will not automatically occur for a load whose input voltage is below a predetermined level, regardless of power supply voltage fluctuations.

In other words, it is possible to detect a load whose power consumption is below a certain level as a small load, almost unaffected by fluctuations in the power supply.

[Brief explanation of the drawing]

Fig. 1 is a schematic circuit diagram of an induction heating cooker according to an embodiment of the present invention, Fig. 2 is a waveform diagram for explaining the principle of small object load detection according to the present invention, and Fig. 3 is a control diagram according to the present invention. A block diagram of the circuit. Figure 4 is a specific circuit diagram of the block in Figure 3. Figure 5 is a waveform diagram of each part of the circuit in Figure 4.
This figure is a circuit diagram showing another embodiment of the control circuit according to the present invention, and FIG. 7 is a waveform diagram of each part of the circuit of FIG. 6. 1... Low frequency AC power supply, 2... Rectifier circuit, 3... Input capacitor, 4... Inverter circuit, 5... Heating Coil (resonant inductor), 6... Control circuit, 61...
Voltage detection circuit, 62... Comparison circuit, 63...
...Oscillation start/stop control circuit, 64...Zero voltage switching circuit, 65...Zero point voltage detection circuit.

Claims (1)

[Claims] 1. A rectifier circuit that converts low-frequency AC power into DC power, an input capacitor and an inverter circuit connected between the DC output terminals of the rectifier circuit, and an induction heating coil connected to the inverter circuit. and a control circuit, the control circuit comprising an input voltage detection circuit that detects the voltage level of the input capacitor voltage, a comparison circuit that operates according to the output signal level of the input voltage detection circuit, and an output signal of the comparison circuit. An induction heating cooker comprising an oscillation start/stop circuit that operates according to the above and controls oscillation start/stop of the inverter circuit. 2. The induction heating cooker according to claim 1, wherein the two-input terminal detection circuit includes a timer circuit that charges and discharges in accordance with the input capacitor voltage level. The 3-input terminal detection circuit consists of a zero-voltage switching circuit including a switching element that turns on and off in the vicinity of AC human power zero voltage, and a circuit that detects a zero-point voltage proportional to the input capacitor terminal voltage by the function of the zero-voltage switching circuit. An induction heating cooker according to claim 1, characterized in that: